#include<iostream>
#include<vector>
#include<queue>
#include<cstring>
#include<cmath>
#include<map>
#include<set>
#include<cstdio>
#include<algorithm>
#define debug(a) cout<<#a<<"="<<a<<endl;
using namespace std;
const int maxn=1e3+100;
typedef long long LL;
inline LL read(){LL x=0,f=1;char ch=getchar();	while (!isdigit(ch)){if (ch=='-') f=-1;ch=getchar();}while (isdigit(ch)){x=x*10+ch-48;ch=getchar();}
return x*f;}
char s[maxn],b[maxn];
bool judge(char str[],LL i,LL j,LL k){
    if(str[i]==str[j]||str[i]==str[k]||str[j]==str[k]) return false;
    else return true;
}
int main(void)
{
  cin.tie(0);std::ios::sync_with_stdio(false);
  LL n,p;cin>>n>>p;
  cin>>(s+1);
  for(LL i=1;i<=n;i++) b[i]=s[i];
  if(n==1){
    bool flag=1;
    if(s[1]=='z'||('a'+p-1)<=s[1]) cout<<"NO"<<"\n";
    else cout<<char(s[1]+1)<<"\n";
    return 0;
  }
  bool flag=1;
  for(LL i=n;i>=1;i--){

     for(char j=s[i]+1;j<='a'+p-1;j++){
        b[i]=j;

        if(i<2||judge(b,i-2,i-1,i)==true){
            flag=0;
            if(i==n){

                for(char u=s[i]+1;u<='a'+p-1;u++){
                    b[n]=u;
                    if(judge(b,n-2,n-1,n)==true) break;
                }
            }
            else{
                for(LL k=i+1;k<=n;k++){
                    for(char u='a';u<='a'+p-1;u++){
                        b[k]=u;
                        if(judge(b,k-2,k-1,k)==true) break;
                    }
                }
            }
        }
        if(flag==0) break;
        b[i]=s[i];
    }
    if(flag==0) break;
  }
  if(1==flag){
    cout<<"NO"<<"\n";
  }
  else{
    for(LL i=1;i<=n;i++) cout<<b[i];
  }
return 0;
}
